Promoter-negative mutations in the integrase operon of bacteriophage lambda will be isolated, by screening lac phenotypes of mutagenized derivatives of a strain in which lacZ is fused to this promoter through the trpB gene. The nucleotide sequence of the normal promoter and of a trp-lambda fusion (Delta303) affecting promoter activity will be determined. Biotin homoenzyme synthetase of Esherichia coli will be purified and tested for its ability to bind to bio DNA and to repress. The structural genes for E. coli biotin sulfoxide reductase will be identified by mapping ts mutants.